Many modern cellular communications systems (i.e., Third Generation Partnership Project (3GPP) Long Term Evolution Advanced (LTE-A) compliant communications systems) use uplink sounding, commonly referred to as sounding reference signals (SRS), from a user equipment (UE) to an access node to allow the access node to obtain an estimate of the uplink channel between the UE and the access node. Uplink sounding is often used in time division duplexed (TDD) communications systems that allow the access node to use channel reciprocity to obtain estimates of a downlink channel between the access node and the UE from the estimated uplink channel.
In communications systems that are link-budget limited (such as communications systems operating above 6 GHz, e.g., millimeter wave (mmWave) communications systems) the reference signals would need to be beamformed (at a transmitting device and a receiving device) to meet performance requirements in an operating environment with high path-loss. Therefore, a UE would have to transmit a large number of SRS symbols in order to facilitate channel estimation between each transmit antenna (or port) and each receive antenna at each access node. The transmissions of the large numbers of SRS would significantly impact overall communications system performance by consuming a large amount of beamformed uplink resources as overhead. This problem is especially a problem in configurations with limited uplink resources.
Multi-point (MP) communications is a communications technique used in many modern cellular communications systems to help improve overall communications system performance, increase resource utilization, and improve user experience. MP communications involves coordinated transmission and/or reception with multiple geographically separated access nodes. Coordination enables joint scheduling and transmissions as well as joint processing of received signals. It is noted that the issues presented above regarding the transmission of large numbers of SRS is significantly magnified in an environment utilizing MP communications.